In recent years, attention has been paid to a solar cell as a clean energy source in light of an upsurge of consciousness of environmental problems. At present, solar cell modules in various forms have been developed and suggested.
Generally the solar cell module is produced by laminating a transparent front substrate, a filler layer, a solar cell element as a photovoltaic device, a filler layer, a backside protective sheet, and the like in this order and then thermally pressure-bonding them by vacuum suction in such as a lamination method.
Today, as material for composing the filler layer for a solar cell module, ethylene-vinyl acetate copolymer resin with a thickness of 100 μm to 1500 μm has most commonly been used in terms of the processibility, layering workability, production cost, and so forth.
However, the filler layer of the ethylene-vinyl acetate copolymer resin is not necessarily sufficient in the adhesion strength to the transparent front substrate or the backside protective sheet and has a problem that the disadvantageous weakness becomes apparent in the long time use in outdoors. Further, in the case of producing a solar cell module using the filler layer of the ethylene-vinyl acetate copolymer resin, the ethylene-vinyl acetate copolymer resin is thermally decomposed and evolves acetic acid gas and the like depending on the conditions of the thermal pressure bonding. These gases not only worsen the working environments but also cause adverse effects on the solar cell element and electrodes to result in deterioration and decrease of electric power generation efficiency.
Therefore, a method of polymerizing a silane compound with the resin is employed as a method for providing the resin, which is a material for the filler layer, with an adhesive property to glass or metals to be used for the transparent front substrate or the backside protective sheet.
Generally, there are two polymerization methods; a copolymerization method and a graft polymerization method. The copolymerization method is a method carried out by mixing monomers, a catalyst, and an unsaturated silane compound and carrying out polymerization reaction at prescribed temperature and pressure. The graft polymerization method is a method carried out by mixing polymers, a free-radical initiator, and an unsaturated silane compound and polymerizing the silane compound to the polymer main chain or side chains in stirring condition at a prescribed temperature.
For example, in order to provide strength, heat resistance and durability to the material itself by causing crosslinking reaction in the resin, which is the material of the filler layer, at the time of thermal pressure bonding, the following methods have been proposed; a method using a resin sheet obtained by adding a silane coupling agent and an organic peroxide to the ethylene-vinyl acetate copolymer resin (Japanese Patent Publication (JP-B; KOKOKU) No. 14111/1987 (i.e., SHO 62-14111)); a method using a resin sheet obtained by adding an organic peroxide to an ethylene-vinyl acetate copolymer resin graft-modified with an organic silane compound (Japanese Patent Publication (JP-B; KOKOKU) No. 9232/1987 (i.e., SHO 62-9232)); and a method using a resin sheet obtained by adding an organic peroxide to a ternary copolymer resin of ethylene-ethylenic unsaturated carboxylic acid ester-ethylenic unsaturated silane compound (Japanese Patent Publication (JP-B; KOKOKU) No. 104729/1994 (i.e., Hei 6-104729)), however since these methods all use the organic peroxide, the organic peroxide is decomposed at the time of sheet formation to induce crosslinking reaction of the resins to make sheet formation difficult or to deteriorate processibility at the time of lamination or the decomposition products derived from the organic peroxide remain in the adhesion interfaces and cause adhesion inhibition at the time of lamination.
There is another problem that the silane compound is expensive and further improvements are still required.